The faintly smiling stone baboons look so modern few could guess they watched the Roman legionaries march into Egypt, the fall of the Roman empire – and almost 2,000 years later – a scandal begin that almost brought down a government.

The unique pair– 2,500 years old and originally made to flank the entrance to a temple of Thoth in ancient Egypt – have been reinstated by the National Trust in the gardens at Cliveden, Buckinghamshire, after an absence of half a century. The gardens around the mansion were the setting for the 1961 Profumo affair, where Christine Keeler, mistress of a Soviet naval attache who was also a spy and John Profumo, the war secretary, fatefully met. Their triangular affair, and Profumo's denial of it to parliament, ruined his career and led to the suicide of Stephen Ward, the society osteopath who introduced them all.

There is nothing like the granite baboons in any garden in Britain, or as far as curator and Egyptologist Wendy Monkhouse can discover, in any garden or museum in the world.

"There are single Egyptian baboons in museums including the British Museum and pairs carved in other stone, but I know of no other pair in granite," she said. "It's such a hard stone, the carving is still as crisp and clean as new. They are wonderfully made – there's something both cuddly and rugged about them."

The monkeys were among shiploads of antiquities imported by William Waldorf Astor, once the richest man in America, later a British subject and media tycoon – he owned the Pall Mall Gazette and the Observer, now the Guardian's sister paper – for his terraced gardens overlooking the Thames.

The garden is still dotted with nymphs, gods, fountains, the entire balustrade of the Villa Borghese in Rome and giant terracotta jars.

The baboons, seen as sacred animals or even incarnations of Thoth, the god of writing, were particularly suitable for a newspaper baron. Monkhouse believes Astor bought them in Rome in 1898, where they may have been brought home from Africa by the Roman armies, who also imported the cult of Thoth and live baboons among many exotics.

Astor later moved to Hever Castle in Kent, leaving his son and wife Nancy to take over Cliveden. In 1942 the estate passed to the National Trust although the family remained in the house, which is now a hotel.

In the 1960s the Astors moved the baboons to another property, but they have now been presented permanently to the trust, which has completed £18,000-worth of restoration.

• This article was amended on 11 October 2010, to restore dropped words that made clear Christine Keeler's relationship to the Soviet naval attache.